The present invention relates to a method of controlling the rotational speed of an internal combustion engine in the idling condition or the decelerating condition.
There is known a method of controlling the rotational speed of an internal combustion engine in the idling condition or the decelerating condition, which involves controlling the flow rate of intake air drawn into the engine when the engine is in the idling condition or the decelerating condition, namely, when a throttle valve disposed in an intake passage of the engine is at the idling position. According to this conventional method, the flow rate of intake air is controlled by adjusting the cross-sectional area of a flow passage or the opening time period of a flow passage by means of a control valve disposed in a bypass passage, which connects a region of the intake passage at a position located upstream of the throttle valve to a region of the intake passage at a position located downstream of the throttle valve.
The control valve is adjusted in accordance with a feedback signal indicating the difference between the detected actual rotational speed of the engine and a desired rotational speed in the idling condition. This feedback control operation of the flow rate of intake air is carried out not only in the idling condition or the decelerating condition of the engine but also in the ordinary driving condition of the engine. A variable range of either the cross-sectional area of the flow passage or, in the case of a cyclically opened and closed flow passage, the time period of each cycle during which the flow passage is opened is ordinarily predetermined. This permits the flow rate of air flowing through the bypass passage to be controlled corresponding to a value within the predetermined variable range of the cross-sectional area or the opening time period of the flow passage irrespective of a difference between the actual rotational speed of the engine and the above-mentioned desired value of the rotational speed.
Generally, when the temperature in an internal combustion engine is low, for example, in the case of the warming-up operation, since the atomization or gasification of the air-fuel mixture is not sufficient and the viscosity of the engine oil is high, driving at idling speed cannot be performed in a stable manner. Therefore, when the engine temperature is lower than a predetermined level, conventional internal combustion engines, are controlled so that the idling rotational speed is forcibly increased by a certain value. This technique is called fast idle control. However, according to this fast idle control, when the engine temperature is lower than a certain value, the rotational speed is increased indiscriminately, which has the disadvantage that an optimum idling rotational speed corresponding to the actual temperature of the engine is not attained.
Furthermore, in the conventional method for controlling the rotational speed of the engine, since the upper limit and lower limit values of the flow rate of intake air to be controlled by controlling the cross-sectional area or the opening time period of the flow passage are always constant irrespective of the engine temperature, following problems are encountered. One is that when the engine temperature is low, the rotational resistance of the engine is high. Accordingly, it is necessary to take in air in a much larger quantity than when the temperature of the engine is high. Contrary to this, in the case where the engine temperature is high and the variable range of the flow rate of air taken in is large, when the engine is decelerated from a low rotational speed that is much lower than the desired rotational speed, the control valve disposed in the bypass passage is controlled to be fully opened. If the load on the engine is abruptly decreased in this state, the rotational speed of the engine can be abruptly increased drastically, and therefore, a very dangerous driving state can occur.